The Yawn is an experimental B.O.W. developed by the Umbrella Corporation in their Arklay Mountains facility and appears as a boss in Resident Evil and its remake. It is a snake that has increased in size and ferocity. During experimentation, the specimen escaped from the laboratory and went into hiding within the mansion above ground. When the T-virus outbreak occured within the facility, the Yawn was infected and increased in size. Large boils appeared on its body that split its scales apart and its venom became much more potent. During the outbreak, the Yawn would feed on the uninfected Umbrella personnel by swallowing them whole.
When the Special Tactics & Rescue Squad (S.T.A.R.S.) entered the Arklay Mansion during their investigation of the cannibalistic murders in Raccoon City, the Yawn hid on the upper floor and attacked Bravo Team member Richard Aiken, severely wounding him and poisoning him. Richard will live or die depending on the player; if Chris Redfield recovers a bottle of serum and gets it to Richard in time, he will recover and be moved to the mansion's infirmary (though he will die later saving Chris in the Aqua Ring). If Jill Valentine brings serum for Richard, he will later come to her rescue during her first encounter with the Yawn, but will be devoured by the creature in the process. The Yawn is first encountered in the mansion's attic and will attack by biting the player, which has a high probability of poisoning. If the player is on low health and the Yawn attacks again, it will either eat the player whole or crush him/her within its coils. After taking enough damage (and eating Richard if Jill is the player), the Yawn will escape through a fireplace.
The Yawn will appear again in the mansion library, crashing through a window and chasing the player down a hall into the main room. Its attack strategy remains the same and their is more room to move, though the Yawn can ambush the player by sliding around the large bookcase. After recieving enough damage, the Yawn will die and in its death-throws bash the bookcase and knock it over.
Umbrella has conducted no further research into snake-based bio-weapons since the Yawn due to its uncoordinated mutations. Its size was inconsistent with its length, producing the large sores that break through its scales.